Cable television (CATV) service providers offer television, data, telephone and other entertainment and useful services to subscribers at the subscriber's premises.
The typical medium for delivering these services is a cable network which is formed by a relatively large number of high-frequency, electrical signal-conducting coaxial conductors or cables, all of which are linked together to distribute the high-frequency signals over a wide geographic area to substantial numbers of geographically-separated subscribers. The high-frequency signals are delivered to television sets, computers, telephones and other subscriber devices, and those subscriber devices convert the information carried by the high-frequency signals into the services that the subscriber desires.
Because of the extensive nature of the cable network, the signals received at the subscriber premises are reduced in strength compared to the strength of the transmitted signals. The amount of signal strength reduction depends on the length of the pathway through the cable network which the signals pass before arriving at the subscriber premises. For this reason, it is typical to provide an amplifier at the subscriber premises to increase or amplify the strength of the signals received from the cable network before delivering the signals to the subscriber devices.
Some types of subscriber devices, such as television sets, deliver better performance in response to receiving amplified signals. Other types of subscriber devices may require non-amplified or passive signals for proper functionality. For example, lifeline telephone service operates on the basis of passive signals received at the subscriber premises, because the functionality of such telephone service cannot depend on the proper functionality of an amplifier or other active signal conditioner in the signal path. A failed or inoperative amplifier or other active device in the signal path could completely terminate telephone communications, which could be dangerous in emergency situations.
Passive-active network interface devices have been developed to provide both passive and active, i.e. amplified, signals at the subscriber premises for the two different types of subscriber devices which operate from passive and active signals. Such passive-active network interface devices include a signal splitter which essentially divides or branches the incoming, or “downstream,” signals from the cable network into passive and active branches. The passive branch downstream signals are conducted through a passive branch of the network interface device without amplification or modification and applied to those subscriber devices which require passive signals for operation, such as, for example, a voice modem for a telephone set. The active branch downstream signals are conducted to an active signal conditioner, such as an amplifier, of an active branch of the network interface device. The active signal conditioner amplifies the strength of the signals or modifies some characteristic of the signals before the amplified, or conditioned, signals are delivered to one or more subscriber devices. The amplified signals are applied to those subscriber devices that benefit from the amplified signals, such as a television sets and computers.
The high-frequency signals conducted through the cable network are susceptible to distortion from a number of sources. It is for this reason that coaxial cables are widely used to shield the high-frequency signals from degrading influences of the ambient environment. One requirement for maintaining high-quality signal conduction in a coaxial cable is properly terminating the coaxial cable. An improper termination causes reflections of the incident signals back into the transmission path. The reflections cause degradation of the desired incident signals received by the subscriber. The degradations are exemplified by amplitude ripple, group delay ripple, latency, and other similar effects which distort or reduce the incident signals. The signal reflections cause the subscriber to experience a degraded quality of service, or in some cases the level of degradation may be so severe as to prevent the subscriber from receiving meaningful service.